WHITE REFLECTANCE

STANDARD

SOL MARIE CRUZ RIVERASeptember 19,2006

INEL6088

WHAT IS WHITE?

An object can be called “white” when, for all the colors (wavelengths) of the visible spectrum, 100% of the light shining on the object is reflectively diffused.

REFLECTIVELY DIFFUSED

Diffused Ideal White

Reflectively diffused mean that light is reflected back uniformly in all directions.

WHITE REFLECTANCE STANDARD 99% Ideal for Calibration and Standardization Durable and Waterproof Highly Diffuse and Lambertian Consistent Reflectance Over Time Thermally Stable Specifications:

Overall Dimensions: 1.50” Dia. x 0.55”T Reflectance Area: 1.25” Dia . Max. Operating Temp.: 400°C Diffuse Reflection Std: ±4% over 200-2500nm

CALIBRATE AN IMAGE – VALUE OF THE CORRECTED PIXEL

blackave

whiteave

blackave

pixelreal

pixelcorrected VV

VV

V

255

whiteaveV Average value of the white

reflectance target

blackaveV Average value of the

black target

pixelrealV Value of a specific pixel of

the reference image.

0 whiteaveV _blackaveV _

255

pixelrealV _

pixelcorrectedV _

HOW TO CALIBRATE AN IMAGE USING THE WHITE REFLECTANCE STANDARD Take an image of the white reflectance standard

and a black object. Take the average of the pixels of the white

reflectance standard. Take the average of the pixels of the black

object. Subtract the average of the pixels of the black

object to all the pixels of the Reference Image. Multiply all the pixels of the resultant image by

the ratio : blackave

whiteave VV

255

REFERENCE IMAGE

Figure 1: Reference Image

HISTOGRAM REF. IMAGE

Figure 2: Histogram of the Reference Image

AVERAGE VALUES-REF.IMAGE

0.2294arg

arg

ettblacktotal

pixelsettblackVblackave

230.78arg

arg

ettwhitetotal

pixelsettwhiteVwhiteave

CALIBRATED IMAGE

Figure 3: Calibrated Image

HIST CORRECTED IMAGE

Figure 4: Histogram of the Corrected Image

AVERAGE VALUES-NEW.IMAGE

0.2465arg

arg

ettblacktotal

pixelsettblackV

newblackave

253.37arg

arg

ettwhitetotal

pixelsettwhiteV

newwhiteave

REAL vs. CORRECTED IMAGE

The white is more brilliant in the corrected image.

0.2465 253.37 0.2294 230.78 newblackave

newwhiteave

blackave

whiteave VVVV

UNDERSTANDING WHITE BALANCE – DIGITAL WORLD

We can use filters to correct for incandescent or fluorescent lighting.

In the digital world these correction filters are replaced by a feature called “White Balance”.

WHITE BALANCE

The purpose of the WB is to make sure that the imaged white corresponds to the required white

LIGHT COLOUR TEMPERATURE The reason that pictures turn out with a

yellow/orange cast in incandescent (tungsten) lighting and bluish in fluorescent lighting is because light has a colour temperature.

Low colour temp shifts light toward the red. A high colour temp shifts light toward the

blue. Different light sources emit light at different

colour temperatures.

MANUAL WHITE BALANCE

Tell the camera which object in the room is white and supposed to come out white in the picture. Point to a white object, filling the screen completely with it.

The camera calculate the difference between the current colour temperature of that object and the correct colour temperature of a white object.

Then shift all colours by that difference.

PRESET WHITE BALANCE

Cameras provide preset white balance settings such as: Tungsten, Fluorescent, Cloudy, Sunny, etc.

Preset WB can improve the image specially under indoors lighting.

AUTO WHITE BALANCE

Selected by default Appropriate for general snapshots, and

where the illumination is principally from a single type.

AWB works very well in sunny and cloudy outdoors.

Works fine for most indoors situations.

EXAMPLES

Auto White Balance

Preset White Balance - Tungsten

•Indoor lighting: two incandescent (tungsten) bulbs.

•By a preset Tungsten WB, the image gives a truer representation of the scene. Fan is really white.

© Danny Pascale and The Babel Color Company

REFERENCES

[1]Edmund Optics, 2005 Optics and Optical Instruments Catalog

[2] http://www.gigahertzoptik.com [3] http://www.photoxels.com/tutorial_white-

balance.html [4] http://www.BabelColor.com